Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. need a source of dc power. Because power is generally delivered through a wall outlet as high-voltage ac power, a device, typically referred to as a power supply, is required to convert the high-voltage ac power to usable dc power for many electrical devices. Moreover, the power supply often must provide a type of electrical isolation between the source of high voltage ac power and the dc power to meet the requirements of safety agencies. The usable dc power may be provided by the power supply directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. In operation, a power supply may use a controller to regulate output power delivered to an electrical device that may be generally referred to as a load. The controller regulates the transfer of energy to the load. In one instance a controller may control a power switch to turn on and off in response to feedback information from a sensor to transfer energy pulses to the output from the high-voltage ac power source.
Every conductor in a power supply is electrically coupled to the space external to the power supply through an electric field. There is a difference in voltage between any two points in an electric field. Therefore, there is a voltage between every conductor in the power supply and an arbitrary reference location outside the power supply that is often referred to as earth ground, sometimes referred to simply as earth, or as ground. The voltage between a conductor and earth may be positive, negative, or zero.
The coupling of the electric field and the associated voltage is typically represented as stray capacitance in an electric circuit. When the voltage between a conductor and earth changes value, it creates a displacement current in the stray capacitance that couples the conductor to earth. A large rate of change in the voltage can produce a substantial displacement current. The current is referred to as displacement current to distinguish it from conduction current. A displacement current is a changing electric field in space that is equivalent to a movement of electric charge in a conductor. Current that is a movement of charge in a conductor is referred to as conduction current.
A dc current has a constant value with respect to time. In contrast, an ac current is a value that varies with time. A current in general can be the sum of a dc current and an ac current. Conduction current can be the sum of a dc current an ac current. However, a displacement current is only an ac current because an ac current is equivalent to a changing electric field.
Electric current flows in a closed path. In other words, for every current leaving a location there must be a current of the same magnitude returning to the same location. The rule of the closed path holds for both displacement current and for conduction current. The closed path of a current can include both displacement current and conduction current.
A power supply typically must limit noise current in its input conductors to meet the limits specified by regulatory agencies. Current that has the same magnitude and direction (toward the power supply or away from the power supply) in two or more conductors at the same time is called common mode current. Current that has the same magnitude but opposite directions in two conductors is called differential current.
The common mode current in the input conductors is generally a noise current that does not contribute to the power received by the power supply, whereas the differential current provided by the input voltage source delivers the power received by the power supply. Common mode current originates chiefly from the fast switching of high voltage in the power supply. The displacement current that is created by the changing voltage returns to its place of origin on a path that includes the input conductors of the power supply, and therefore contributes to the noise current that is limited by regulatory agencies.
One way to reduce the common mode current is to place inductive components in the input conductors. These components are sometimes referred to as common mode inductors or as common mode chokes. A common mode inductor has two or more windings on a common magnetic core where the windings are configured to oppose common mode noise currents that would flow in the same direction in the input conductors while offering negligible opposition to differential currents that provide power to the power supply. A preferred alternative to the use of common mode inductors in the input conductors is to add special windings to an energy transfer element that is already in the power supply for power conversion purposes.
The energy transfer element in the power supply, sometimes called a transformer, is an inductive component with multiple windings on a magnetic core. During operation the transformer allows the transfer of energy between an input side (referred to as a primary side) of the power supply and an output side (referred to as the secondary side) of the power supply. The transformer also provides galvanic isolation between the input and an output of the power supply. Galvanic isolation is a property that prevents dc current from flowing between an input conductor and an output conductor. A winding necessary for power conversion is a power winding. “Special windings” are additional windings that do not take part in the power conversion function. A winding that may provide both shielding functions and power conversion functions, such as for example a bias winding that provides a bias voltage to operate a component of a control circuit, is considered a power winding, not a special winding.
The special windings are often referred to as balance windings and cancellation windings. They are sometimes included in the general category of shield windings that distinguish them from the power conversion windings that are required for the power supply to operate. The purpose of the special windings is to restrict the displacement current to a path that does not include the input conductors of the power supply. It is preferred that displacement current remains within the energy transfer element, and that the equivalent conduction current does not go very far beyond the terminals of the energy transfer element. The special windings accomplish their purpose by introducing electric fields at the proper place and in the proper strength to steer the displacement current to take a desired path.
Well-known methods have been developed to design and to construct energy transfer elements that contain shield windings for the purpose of reducing common mode current in power supplies. These methods encounter difficulties when a winding of the energy transfer element has a small number of turns. The conventional methods are most effective when the shield windings have an integral number of turns that is close to the number of turns of the power conversion windings.
In applications to power supplies where the ratio of input voltage to output voltage is very large or very small, a power winding can have as few as one or two turns. In such situations, it may be impossible to give a conventional shield winding the number of turns necessary to achieve the desired reduction in displacement current. If the shield winding produces an electric field that is too small, the winding will not be very effective. If the shield winding produces an electric field that is too large, the winding can cause the common mode current to increase instead of decrease.